1. Field of the Invention
The invention relates to thermal viewers or infrared imaging devices, particularly a militarized system using common modules in the range of wavelengths from 2 to 14 microns.
2. Description of the Prior Art
Thermal imagers have evolved mainly along two lines. The first used vacuum tube technology, specifically vidicon and orthicon camera tubes with special retinas sensitive to infrared. The pyroelectric vidicon is an even more specialized version of this group. The second approach uses solid state devices usually in the form of diodes as individual pixel detectors. Since these diodes have been expensive to produce such viewers have slowly evolved from a model with a single diode over which the infrared image is scanned in two orthogonal directions to the more common single row of detectors centered on and normal to the optical axis of the viewer which requires scanning in one direction only. Two dimensional arrays are now emerging in devices that are called "staring detectors." The single row type usually employs a vibrating mirror to scan the image although there are designs with moving detectors and light emitting diodes that are electrically and mechanically intercoupled. The staring type detectors most advantageously employ the charge-coupled-device (CCD) sometimes referred to as "bucket brigade" circuits, which can also be used with row type detectors. As will be immediately evident to those skilled in the art, the present invention is applicable to all of the above described systems, but for simplicity it will be completely described only in combination with the more prevalent common module system with a single row of diodes, which is currently in production.
The common module system consists of a cubic central scanner module which is surrounded on its four windowed sides by add on modules. The top and bottom walls of the scanner module are used to mount a scanning mirror and its drive mechanism. In some models opposite sides of the mirror are used for synchronized scanning of infrared and visible images. A pair of opposed modules contain an objective lens for the infrared side of the mirror and an eyepiece for the visible image side. The remaining pair of add on modules contain the infrared detectors and electroluminescent devices with their accompanying coolers and intercoupling electronic circuitry. The add on modules can be further modularized as convenience dictates. Precisely defined interface flanges align the modules to a normalized central optical axis so that additional modules can be developed independently and inserted in the system. The light is usually collimated before it enters the central modules to prevent angular dispersion of the rays by the mirror. A more complete description of the common module system appears in U.S. Pat. No. 3,760,181 for a "Universal Viewer for Far Infrared" by Patrick J. Daly et al, issued Sept. 18, 1973.
In operation the above thermal viewer provides an image of varying brightness which is proportional to the temperature of the objects in the field of view. The temperatures are identified by the total radiation from each object over the bandwidth of the diodes and within certain transmission windows imposed by the earth's atmosphere. A number of these windows exist in the 2-14 micron range. The source of the radiation is usually a blackbody and will have the usual peak radiation frequency, gradual decay at lower frequencies and upper frequency cutoff. Other targets such as a gas flame or a gas present in the transmission path from a high temperature source will exhibit an emission or absorption spectrum characterized by narrow bands of radiation (or their absence) of different intensity. Such targets normally cannot be separated from the large amount of background radiation present on the basis of total radiation.